Processing input/output requests using proxy and owner storage systems

ABSTRACT

A first storage system is configured as a proxy for a logical volume stored on a second storage system in a distributed computing environment. A probe request verifying availability of the logical volume is conveyed to an identified port, and upon receiving a response from a second storage system verifying the availability of the logical volume for an I/O request, the I/O request is conveyed to the identified port, a result of the I/O request is received from the identified port, the result is conveyed to the host computer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/339,906, filed Jul. 24, 2014, which is a Continuation of U.S. patentapplication Ser. No. 13/915,922, filed Jun. 12, 2013, both of which arehereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to storage systems, andspecifically to a storage facility configured to process input/outputrequests via a proxy storage system.

BACKGROUND

In a storage area network (SAN), remote computer storage devices such asdisk arrays can be made accessible to host computers so that the storagedevices appear as if they are locally attached to the host computer'soperating system. SANs may be implemented using Small Computer SystemInterface (SCSI) storage devices, in which SCSI protocol entitiesperform input/output (I/O) operations (e.g., data reads and writes) andare exposed through a unique identifier such as a logical unit number(LUN) on a path. A given LUN typically corresponds to a logical volume,and may be represented within the host computer's operating system as adevice. Interaction with a LUN is initiated by a SCSI initiator port ona host computer, which can issue various I/O request types to the LUN ona target data storage device.

The description above is presented as a general overview of related artin this field and should not be construed as an admission that any ofthe information it contains constitutes prior art against the presentpatent application.

SUMMARY OF THE INVENTION

There is provided, in accordance with an embodiment of the presentinvention a method, including configuring a first storage system as aproxy for a logical volume stored on a second storage system in adistributed computing environment, conveying, to an identified port, aprobe request to verify an availability of a logical volume for an I/Orequest, and upon receiving a response from a second storage systemverifying the availability of the logical volume for the I/O request,conveying the I/O request to the identified port, receiving a result ofthe I/O request from the identified port, and conveying the result tothe host computer.

There is also provided, in accordance with an embodiment of the presentinvention a proxy storage system operating in a distributed computingenvironment, including a proxy port coupled to a storage area network(SAN), and a processor configured to convey, to an identified ownerport, a probe request to verify an availability of a logical volume foran I/O request, and upon receiving, via the proxy port, a response froma owner storage system confirming the availability of the logical volumefor the I/O request, to convey the I/O request to the identified ownerport, to receive a result of the I/O request from the identified ownerport, and to convey the result to the host computer.

There is further provided, in accordance with an embodiment of thepresent invention an owner storage system operating in a distributedcomputing environment, including a storage device configured to store alogical volume, multiple ports configured to communicate with a proxystorage system via a storage area network (SAN), and a processorconfigured, to receive, via one of the ports, a probe request from theproxy storage system to verify an availability of the logical volume foran input/output (I/O) request from a host computer in communication withthe proxy storage system, the logical volume being mapped between thehost computer and the proxy storage system, to verify the availabilityof the logical volume for the I/O request, and subsequent to conveying aresponse to the proxy storage system confirming the availability of thelogical volume for the I/O request, to receive the I/O request from theproxy storage system via the one of the ports, to process the I/Orequest, and to convey a result of the I/O request to the proxy storagesystem via the one of the ports.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a block diagram that schematically illustrates a storagesystem, in accordance with an embodiment of the present invention;

FIG. 2 is a block diagram of a facility comprising multiple storagesystems configured to process proxy input/output (I/O) requests, inaccordance with an embodiment of the present invention;

FIG. 3 is a flow diagram that schematically illustrates a method for aproxy storage controller to process a proxy I/O request for a logicalvolume stored on an owner storage controller, in accordance with anembodiment of the present invention; and

FIG. 4 is a flow diagram that schematically illustrates a method for anowner storage controller to process a proxy I/O request received from aproxy storage controller, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

There may be instances when a storage administrator wants to migrate thelogical volume from a first storage system to a second storage system inorder to balance the storage utilization across the storage systems.Embodiments of the present invention provide methods and mechanisms forseamlessly migrating the logical volume from the first storage system tothe second storage system. As explained hereinbelow, after copying thelogical volume to the second storage system, the first storage systemcan be configured as a proxy for the logical volume that is now storedon the second storage system, thereby enabling the first storage systemto continue to receive and process input/output (I/O) requests for thelogical volume. In embodiments described herein the first storage systemmay also be referred to as a proxy storage controller and the secondstorage controller may also be referred to as an owner storagecontroller, wherein the proxy and the owner storage controllers compriseSmall Computer System Interface (SCSI) based storage systems thatcommunicate over a multipath Small Computer System Interface (SCSI)based storage area network (SAN).

FIG. 1 is a block diagram that schematically illustrates a dataprocessing storage subsystem 20, in accordance with an embodiment of theinvention. The particular subsystem (also referred to herein as astorage system) shown in FIG. 1 is presented to facilitate anexplanation of the invention. However, as the skilled artisan willappreciate, the invention can be practiced using other computingenvironments, such as other storage subsystems with diversearchitectures and capabilities.

Storage subsystem 20 receives, from one or more host computers 22,input/output (I/O) requests, which are commands to read or write data atlogical addresses on logical volumes. Any number of host computers 22are coupled to storage subsystem 20 by any means known in the art, forexample, using a network. Herein, by way of example, host computers 22and storage subsystem 20 are assumed to be coupled by a Storage AreaNetwork (SAN) 26 incorporating data connections 24 and Host Bus Adapters(HBAs) 28. The logical addresses specify a range of data blocks within alogical volume, each block herein being assumed by way of example tocontain 512 bytes. For example, a 10 KB data record used in a dataprocessing application on a given host computer 22 would require 20blocks, which the given host computer might specify as being stored at alogical address comprising blocks 1,000 through 1,019 of a logicalvolume. Storage subsystem 20 may operate in, or as, a SAN system.

Storage subsystem 20 comprises a clustered storage controller 34 coupledbetween SAN 26 and a private network 46 using data connections 30 and44, respectively, and incorporating adapters 32 and 42, againrespectively. In some configurations, adapters 32 and 42 may comprisehost bus adapters (HBAs). Clustered storage controller 34 implementsclusters of storage modules 36, each of which includes an interface 38(in communication between adapters 32 and 42), and a cache 40. Eachstorage module 36 is responsible for a number of storage devices 50 byway of a data connection 48 as shown.

As described previously, each storage module 36 further comprises agiven cache 40. However, it will be appreciated that the number ofcaches 40 used in storage subsystem 20 and in conjunction with clusteredstorage controller 34 may be any convenient number. While all caches 40in storage subsystem 20 may operate in substantially the same manner andcomprise substantially similar elements, this is not a requirement. Eachof the caches 40 may be approximately equal in size and is assumed to becoupled, by way of example, in a one-to-one correspondence with a set ofphysical storage devices 50, which may comprise disks. In oneembodiment, physical storage devices may comprise such disks. Thoseskilled in the art will be able to adapt the description herein tocaches of different sizes.

Each set of storage devices 50 comprises multiple slow and/or fastaccess time mass storage devices, herein below assumed to be multiplehard disks. FIG. 1 shows caches 40 coupled to respective sets of storagedevices 50. In some configurations, the sets of storage devices 50comprise one or more hard disks, which can have different performancecharacteristics. In response to an I/O command, a given cache 40, by wayof example, may read or write data at addressable physical locations ofa given storage device 50. In the embodiment shown in FIG. 1, caches 40are able to exercise certain control functions over storage devices 50.These control functions may alternatively be realized by hardwaredevices such as disk controllers (not shown), which are linked to caches40.

Each storage module 36 is operative to monitor its state, including thestates of associated caches 40, and to transmit configurationinformation to other components of storage subsystem 20 for example,configuration changes that result in blocking intervals, or limit therate at which I/O requests for the sets of physical storage areaccepted.

Routing of commands and data from HBAs 28 to clustered storagecontroller 34 and to each cache 40 may be performed over a networkand/or a switch. Herein, by way of example, HBAs 28 may be coupled tostorage modules 36 by at least one switch (not shown) of SAN 26, whichcan be of any known type having a digital cross-connect function.Additionally or alternatively, HBAs 28 may be coupled to storage modules36.

In some embodiments, data having contiguous logical addresses can bedistributed among modules 36, and within the storage devices in each ofthe modules. Alternatively, the data can be distributed using otheralgorithms, e.g., byte or block interleaving. In general, this increasesbandwidth, for instance, by allowing a volume in a SAN or a file innetwork attached storage to be read from or written to more than onegiven storage device 50 at a time. However, this technique requirescoordination among the various storage devices, and in practice mayrequire complex provisions for any failure of the storage devices, and astrategy for dealing with error checking information, e.g., a techniquefor storing parity information relating to distributed data. Indeed,when logical unit partitions are distributed in sufficiently smallgranularity, data associated with a single logical unit may span all ofthe storage devices 50.

While such hardware is not explicitly shown for purposes of illustrativesimplicity, clustered storage controller 34 may be adapted forimplementation in conjunction with certain hardware, such as a rackmount system, a midplane, and/or a backplane. Indeed, private network 46in one embodiment may be implemented using a backplane. Additionalhardware such as the aforementioned switches, processors, controllers,memory devices, and the like may also be incorporated into clusteredstorage controller 34 and elsewhere within storage subsystem 20, againas the skilled artisan will appreciate. Further, a variety of softwarecomponents, operating systems, firmware, and the like may be integratedinto one storage subsystem 20.

Storage devices 50 may comprise a combination of high capacity hard diskdrives and solid state disk drives. In some embodiments each of storagedevices 50 may comprise a logical storage device. In storage systemsimplementing the Small Computer System Interface (SCSI) protocol, thelogical storage devices may be referred to as logical units, or LUNs.While each LUN can be addressed as a single logical unit, the LUN maycomprise a combination of high capacity hard disk drives and/or solidstate disk drives.

Examples of adapters 32 and 42 include switched fabric adapters such asFibre Channel (FC) adapters, Internet Small Computer System Interface(iSCSI) adapters, Fibre Channel over Ethernet (FCoE) adapters andInfiniband™ adapters.

FIG. 2 is a block diagram of a facility 60 configured to process proxyinput/output requests, in accordance with an embodiment of the presentinvention. In the description herein, storage controllers 34 and theirrespective components may be differentiated by appending a letter to theidentifying numeral, so that facility 60 comprises host computer 22 andstorage controllers 34A and 34B that are configured to communicate witheach other via SAN 26. In embodiments herein, storage controller 34A mayalso be referred to as a first storage controller 34 or as a proxystorage controller 34, and storage controller 34B may also be referredto as a second storage controller 34 or an owner storage controller 34.

Host computer 22 communicates with SAN 26 via ports 62. Module 36comprises a processor 64 and a memory 66, and communicates with SAN 26via ports 68. In some embodiments ports 62 and 68 may comprise SCSIports, and the SCSI ports may be configured within module 36. Inembodiments herein, ports 68A may also be referred to as proxy ports andports 68B may also be referred to as owner ports.

While for purposes of illustrative simplicity, the configuration in FIG.2 shows module 36 comprising a single storage device 50 storing a singlelogical volume 70, module 36 typically comprises multiple storagedevices 50 storing multiple logical volumes 70. Additionally, a givenlogical volume 70 may be stored across multiple storage devices 50 in agiven storage controller 34.

In embodiments of the present invention, processor 64A executes, frommemory 66A, a proxy layer 72 that enables processor 64A to receive, fromhost computer 22, an I/O request for volume 70B (also referred to hereinas a request to perform an I/O operation on volume 70B), to convey theI/O request to the owner storage controller, to receive a response forthe I/O request from the owner storage controller, and to convey theresponse to the host computer. Processor 64B executes, from memory 66B,an owner layer 74 that enables processor 64B to receive, from the proxystorage controller, an I/O request from host computer 22 for volume 70B,to process the I/O request, and to convey a response to the I/O requestto the proxy storage controller. In embodiments herein, an I/O requestthat storage controller 34A receives from host computer 22 for volume70B that that is forwarded to storage controller 34B may also bereferred to as a proxy I/O request.

Processor 64 typically comprises a general-purpose central processingunit (CPU), which is programmed in software to carry out the functionsdescribed herein. The software may be downloaded to module 36 inelectronic form, over a network, for example, or it may be provided onnon-transitory tangible media, such as optical, magnetic or electronicmemory media. Alternatively, some or all of the functions of processor64 may be carried out by dedicated or programmable digital hardwarecomponents, or using a combination of hardware and software elements.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system”.Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Python, Java, Smalltalk, C++ or the like and conventionalprocedural programming languages, such as the “C” programming languageor similar programming languages. The program code may execute entirelyon the user's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/actions specifiedin the flowchart and/or block diagram block or blocks. These computerprogram instructions may also be stored in a computer readable mediumthat can direct a computer, other programmable data processingapparatus, or other devices to function in a particular manner, suchthat the instructions stored in the computer readable medium produce anarticle of manufacture including instructions which implement thefunctions/actions specified in the flowchart and/or block diagram blockor blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/actions specified in the flowchart and/orblock diagram block or blocks.

Proxy Storage Controller I/O Request Processing

In embodiments of the present invention, storage controller 34A systemcan be configured as a proxy for logical volume 70B that is stored onthe storage controller 34B. In operation, volume 70B is mapped betweenhost computer 22 and storage controller 34A, even though volume 70B isphysically stored on storage controller 34B.

FIG. 3 is a flow diagram that schematically illustrates a method forstorage controller 34A to process a proxy I/O request received from hostcomputer 22, in accordance with an embodiment of the present invention.In a first receive step 80, processor 64A receives, from host computer22, an I/O request for volume 70B, and processor 64A configures the I/Orequest as a proxy I/O request upon determining that volume 70B isstored on storage controller 34B.

In a first identification step 82, processor 64A identifies an initialport 68B on storage controller 34B for processing the I/O request. Insome embodiments the initial port comprises the least busy port 68B. Ina first convey step 84, processor 64A conveys a probe request to initialport 68B to verify an availability of volume 70B for the I/O request.For example, volume 70B may currently be reserved by a different hostcomputer 22, or volume 70B may have a read-only status and the I/Orequest may be for a write operation.

The probe request typically includes a header such as a SCSI commanddescription block (CDB). In some embodiments, processor 64A can splitthe I/O request into multiple sub-requests, and the probe request mayinclude a count of the sub-requests. Splitting an I/O request intomultiple sub-requests is described in more detail in U.S. patentapplication “Load Balancing Input/Output Operations Between TwoComputers”, referenced above.

Documents incorporated by reference in the present patent applicationare to be considered an integral part of the application except that tothe extent any terms are defined in these incorporated documents in amanner that conflicts with the definitions made explicitly or implicitlyin the present specification, only the definitions in the presentspecification should be considered.

Additionally, since the I/O request can be divided into multiplesub-requests, the probe request also enables processors 64A and/or 64Bto detect when the I/O operation indicated by the I/O request iscomplete, and the volume is consistent. For example, prior to taking asnapshot of volume 70B, processor 64 can verify that any pendingsub-requests are completed, thereby ensuring volume integrity.

In a first decision step 86, if processor 64A receives a response fromprocessor 64B indicating an availability of logical volume 70B for theI/O request, then in second convey step 88, processor 64A startsconveying the proxy I/O request to initial port 68B. In embodimentswhere processor 64A splits the I/O request into multiple sub-requests,processor 64A can start sending each of the sub-requests to initial port68B. In the example described in the flow diagram shown in FIG. 3,processor 64A conveys a probe request prior to conveying thesub-requests. In some embodiments, processor 64A can incorporate theprobe request into the first sub-request conveyed to processor 64B.

In a second decision step 90, if processor 64A does not detect a failureof initial port 68B while conveying the I/O request, then in a thirdconvey step 92, processor 64A completes conveying the I/O request to theinitial port. For example, in embodiments where processor 64A splits theI/O request into multiple sub-requests, processor 64A completesconveying all the sub-requests.

In a second receive step 94, processor 64A receives a result of the I/Orequest from initial port 68B. For example, if the I/O request comprisesa read data request, then the response may include data read from volume70B. Likewise, if the I/O request comprises a write data request, thenthe response may include an acknowledgement that the data was writtensuccessfully to logical volume 70B. Finally, in a fourth convey step 96,processor 64A conveys the result of the I/O request to the hostcomputer, and the method ends.

Returning to step 90, if processor 64A detects a failure of initial port68B while conveying the I/O request, then in a third identification step98, processor 64A identifies a non-conveyed portion of the I/O request.For example, in embodiments where processor 64A splits the I/O requestinto multiple sub-requests, upon detecting a failure of initial port68B, processor 64A can identify any non-conveyed sub-requests (i.e.,sub-requests that are still waiting to be conveyed to processor 64B).

In a third identification step 100, processor 64A identifies asubsequent port 68B on storage controller 34B. In embodiments herein,initial port 68B may also be referred to as first port 68B, thesubsequent port 68B may also be referred to as second port 68B. Asdescribed supra when identifying the initial port, the subsequent portmay comprise the least bust port 68B. In a fourth convey step 102,processor 64A conveys a continuation probe to subsequent port 68B. Insome embodiments, the continuation probe is similar to the probe requestconveyed in step 84 in the sense that it initializes a context onstorage controller 34B for receiving sub-requests. However, thecontinuation probe may skip any validity checks (e.g., checking forreservations) for processing the I/O request.

In a fifth convey step 104, upon receiving an response from processor64B indicating that the continuation probe was received, processor 64Aconveys the non-completed portion (e.g., the identified non-conveyedsub-requests) of the I/O request to subsequent port 68B, receives, in athird receive step 106, the result of the I/O request from thesubsequent port, and the method continues with step 96. The response tothe continuation probe verifies successful connectivity to storagecontroller 34A, thereby setting up a context for an atomic I/O operationcomprising the non-completed portion of the I/O request.

Upon a failure of the initial port, there may still be I/O requests (orresponses to I/O requests) pending on the initial port. In someembodiments the continuation probe can “clean-up” any pendingsub-requests still pending on the initial port. In an alternativeembodiment, processor 64A can convey a separate message to processor 64Bto perform a clean-up on the initial port.

Returning to step 86, if processor 64A receives a response fromprocessor 64B indicating that logical volume 70B is not available forthe I/O operation, then processor 64A conveys a message indicating thenon-availability of volume 70B to the host computer, and the methodends.

Owner Storage Controller I/O Request Processing

FIG. 4 is a flow diagram that schematically illustrates a method forstorage controller 34B to process a proxy I/O request received fromstorage controller 34A, in accordance with an embodiment of the presentinvention. In first receive step 110, processor 64B receives, via aninitial port 68B, a probe request to verify an availability (asdescribed supra) of logical volume 70B for processing an I/O requestfrom host computer 22.

In a first decision step 112, if logical volume 70B is available for theI/O request, then in a first convey step 114, processor 64B conveys, viainitial port 68B, a message confirming the volume availability for theI/O request. In a second receive step 116, processor 64B startsreceiving, via initial port 68B the proxy I/O request from processor64A. In embodiments where processor 64A splits the proxy I/O requestinto multiple sub-requests, receiving the proxy I/O request comprisesreceiving the multiple sub-requests, and using information included inthe probe request to “re-assemble” the sub-parts into the proxy I/Orequest.

In a second comparison step 118, if the proxy I/O request comprisesmultiple sub-requests, and processor 64B receives a continuation probeon a subsequent port 68B (different than initial port 68B) prior toreceiving all the sub-requests, then in a third receive step 120,processor 64B completes receiving the proxy I/O request via subsequentport 68B. In a first processing step 122, processor 64B processes theproxy I/O request via subsequent port 68B, and the method ends.

For example, if the proxy I/O request comprises a request to read datafrom logical volume 70B, then processor 68B can retrieve the data fromthe logical volume, and convey the retrieved data to processor 64B viasubsequent port 68B.

While the example shown in FIG. 4 describes a single failure of an ownerport while processing a set of sub-requests (i.e., for a single I/Orequests), a failure of two or more ports is considered to be within thespirit and scope of the present invention. For example, processor 64Amay detect a failure of the subsequent port, and an additionalcontinuation probe can be conveyed to a further (i.e., a third) ownerport 64B to complete processing the I/O request using embodimentsdescribed herein.

Returning to step 118, if processor 64B does not receive a continuationprobe while receiving the proxy I/O request, then in a fourth receivestep 124, processor 64B completes receiving the proxy I/O request viainitial port 68B. In a second processing step 126, processor 64Bprocesses the proxy I/O request via the initial port in and the methodends.

Returning to step 112, if logical volume 70B is not available for theproxy I/O request, then in a second convey step 128, processor 64Bconveys a non-availability message to processor 64A (i.e., in responseto the probe request), and the message ends.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the Figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

What is claimed is:
 1. A method for configuring a first storage system as a proxy for a logical volume stored on a second storage system in a distributed computing environment, comprising: conveying, to an identified port, a probe request to verify an availability of the logical volume for an input/output (I/O) request; and upon receiving a response from a second storage system verifying the availability of the logical volume for the I/O request, conveying the I/O request to the identified port, receiving a result of the I/O request from the identified port, and conveying the result to a host computer.
 2. The method of claim 1, further comprising receiving, by the first storage system, the I/O request from the host computer for the logical volume, wherein the host computer identifies the port on the second storage system for the I/O request.
 3. The method according to claim 1, further including, upon receiving a response from the second storage system verifying the availability of the logical volume for the I/O request, conveying the I/O request to the identified port, receiving a result of the I/O request from the identified port, and conveying the result to the host computer; wherein the I/O request is selected from a list comprising a request to read data from the logical volume and a request to write data to the logical volume.
 4. The method according to claim 1, and comprising receiving, from the second storage system, a response to the probe request indicating a non-availability of the logical volume for the I/O request, and conveying a message to the host computer a message indicating the non-availability.
 5. The method according to claim 2, wherein the second storage system has multiple ports, and identifying the port comprises identifying a least busy port of the multiple ports.
 6. The method according to claim 4, wherein the multiple ports comprise Small Computer System Interface (SCSI) ports coupled to a storage area network (SAN).
 7. The method according to claim 1, wherein conveying the I/O request to the identified port comprises splitting the I/O request into multiple sub-requests, and conveying the multiple sub-requests to the identified port.
 8. The method according to claim 6, wherein the identified port comprises a first port, and comprising upon detecting, a failure in the first port while conveying the multiple sub-requests, identifying any non-conveyed sub-requests, identifying a second port on the second storage system for the I/O request, conveying the identified sub-requests to the second port, and upon receiving a result of the I/O request from the second port, conveying the result to the host computer.
 9. A proxy storage system operating in a distributed computing environment, comprising: a proxy port coupled to a storage area network (SAN); and a processor configured: to convey, to an identified owner port, a probe request to verify an availability of the logical volume for an I/O request; and upon receiving, via the proxy port, a response from an owner storage system confirming the availability of the logical volume for the I/O request, to convey the I/O request to the identified owner port, to receive a result of the I/O request from the identified owner port, and to convey the result to a host computer; wherein the I/O request is selected from a list comprising a request to read data from the logical volume and a request to write data to the logical volume.
 10. The proxy storage system according to claim 8, wherein the processor is further configured: to receive the input/output (I/O) request from the host computer for a logical volume, the logical volume mapped between the host computer and the proxy storage system, and to identify the owner port on the owner storage system for the I/O request, the owner storage system storing the logical volume.
 11. The proxy storage system according to claim 8, wherein the processor is configured to receive, from the second storage system, a response to the probe request indicating a non-availability of the logical volume for the I/O request, and to convey a message to the host computer a message indicating the non-availability.
 12. The proxy storage system according to claim 8, wherein the second storage system has multiple owner ports, and the processor is configured to identify the owner port by identifying a least busy owner port of the multiple owner ports.
 13. The proxy storage system according to claim 8, wherein the proxy port comprises a Small Computer System Interface (SCSI).
 14. The proxy storage system according to claim 8, wherein the processor is configured to convey the I/O request to the identified owner port by splitting the I/O request into multiple sub-requests, and conveying the multiple sub-requests to the identified owner port.
 15. The proxy storage system according to claim 13, wherein the identified owner port comprises a first owner port, and wherein the processor is configured upon detecting a failure in the first owner port while conveying the multiple sub-requests, to identify any non-conveyed sub-requests, to identify a second owner port on the owner storage system for the I/O request, to convey identified sub-requests to the second owner port, to receive a result of the I/O request from the second owner port, and to convey the result to the host computer.
 16. An owner storage system operating in a distributed computing environment, comprising: a storage device configured to store a logical volume; multiple ports configured to communicate with a proxy storage system via a storage area network (SAN); and a processor configured: to receive, via one of a plurality of ports, a probe request from the proxy storage system to verify an availability of the logical volume for an input/output (I/O) request from a host computer in communication with the proxy storage system, the logical volume being mapped between the host computer and the proxy storage system, and to verify the availability of the logical volume for the I/O request.
 17. The owner storage system according to claim 15, wherein the multiple ports comprise Small Computer System Interface (SCSI) ports.
 18. The owner storage system according to claim 15, wherein the I/O request is selected from a list comprising a request to read data from the logical volume and a request to write data to the logical volume.
 19. The owner storage system according to claim 15, wherein upon detecting that the logical volume is not available for the I/O request, the processor is configured to convey a non-availability message to the proxy storage system in response to the probe request.
 20. The owner storage system according to claim 15, wherein the proxy I/O request comprises multiple sub-requests.
 21. The owner storage system according to claim 19, wherein the one of the ports comprises a first of the ports, and the processor starts receiving the multiple sub-requests via the first port, and wherein upon receiving a continuation port via a second of the ports, the processor is configured to complete receiving the sub-requests via the second port, to reassemble the proxy I/O request from the received sub-requests, and to process the proxy I/O request. 